Melt-processible copolymers of tetrafluoroethylene and hexafluoropropylene (TFE/HFP) have long been known, and can be prepared as described in Bro et al. U.S. Pat. No. 2,946,763. However, the stabilization of the copolymers, through freeing them of volatiles by heating, has remained a problem in the preparation procedures. Bro et al. teaches the stabilization of the copolymers by heating them at 300.degree.-400.degree.C. for 0.15 to 30 hours.
Mallouk et al., U.S. Pat. No. 2,955,099, discloses that a change in melt viscosity may occur during the heat stabilization procedure of Bro et al.; and teaches that to stabilize the copolymers against this melt viscosity change, a small amount of a cationic metal compound should be added.
Schreyer, U.S. Pat. No. 3,085,083 adds still another improvement. Schreyer teaches that if the heat treatment is carried out at 200.degree.-400.degree.C. in the presence of water which is present in an amount of at least 2% by weight of the copolymer environment, stabilization is achieved through endcapping of the copolymer. Preferably, the Schreyer treatment is carried out at 340.degree.-380.degree.C. for 2-5 hours in air containing at least 3 weight percent water vapor. This preferred treatment will be referred to hereinafter as the humid heat treatment.
The procedures for improving the thermal stability of melt-processible TFE/HFP copolymers, as taught by the above-described patents, cause the volatilization of monomer and other gases and result in copolymers that are relatively stable to heat. Such volatilization causes the copolymers to foam and rise like bread dough. This effect can cause overflowing of the equipment used for the humid heat treatment, and consequently the foaming places a limit on the amount of copolymer which can be placed in the equipment. Thus, the foaming can limit the rate of production of the finished heat-stable copolymer.